One GTX980Ti would be roughly equal or better performance to 2-SLI Strix GTX970. Much better performance, actually, since many game engines are not code-optimized to make good use of multiple GPUs. And much better stability since a single GPU card will be a lot easier on your PSU, mobo, and proc.
The games you're playing now will look and perform better with more GPU either way. But you might want to consider system requirements for games you'll be playing another year or two down the road, I think the single GTX980Ti would serve you better (plus you could eventually pair it with another GTX980Ti, of course).
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[/Korth]
